I'm quite not sure if both of your basic questions have not yet been answered on the site. Therefore I'll refer in my answer to some other posts where I'm aware of at least partial answers!
To what extent corrupted
Basically the Quran is the source which says that these books have been corrupted there are statements in it which are very clear and there are general statements on the corruption (for example 2:75-76). But no single statement clearly saying that psalm x or verse y etc. in the Bible has been corrupted. So the message of the Quran is/was that some priests kept changing the original scripture for years/decades/centuries and therefore changed the rules of God (shari'a). Maybe this already covers your other Question: What is the basis for the claim that the Prophets, Psalms, Torah and Gospel have been corrupted?
One could basically say that anything which sounds unjust in the former scripture can't be the words of Allah:
Let's say for example (I've never read the bible, but might have heard a few verses/psalms ... so don't expect me pointing at something in special) if you find any verse which encourages injustice, like asking people to give money to Priests to get close to God.
The Quran says that Muhammad's prophecy is present in the scripture of the people of the book.
The Quran says that Jesus has not been crucified.
The Quran presents the fact that interests were haram (prohibited) to Jews but they have re-interpreted the orders of God to make them halal (allowed) if the money transaction isn't between Jews themselves:
For wrongdoing on the part of the Jews, We made unlawful for them [certain] good foods which had been lawful to them, and for their averting from the way of Allah many [people], (160)
And [for] their taking of usury while they had been forbidden from it, and their consuming of the people's wealth unjustly. And we have prepared for the disbelievers among them a painful punishment. (4:160-161)
On the whole the Quran covers as it feels to one third (or maybe more) the history, rulings etc. of the banu Israel. Beside this you'll find a few stories like the one of the 7 sleepers (which is related to Christians) especially in surat al-Kahf (18) and the biography and some teachings of Jesus which are rather related to Christians.
So basically the Muslim perspective is if any rulings or stories (historical facts?) in the scripture of the people of the book (Jews and Christians) is in opposition to what the Quran says it must be corrupted. Some Islamic schools of Jurisprudence therefore take rulings from the people of the scripture on the other hand as valid rulings for Muslims if there's no rule (Quran text or hadith report) saying that it was abrogated!
Note that the Quran didn't cover all the stories which you may find in the Bible (An example might be Did the events at the Tower of Babel occur?) as the major intention -as it seems- for telling stories of former nations was to use them as examples. Therefore some ahadith (reports of the Prophet Muhammad()) allow spreading knowledge of the people of the book which isn't clearly opposing the Quran and sunnah.
Also read these posts:
Which part of the Christian Bible is corrupted?
If we had access to the original Bible, would we still need the Qur'an?
What is the Injil that is referred to in Qur'an?
Did the companions of Prophet Isa (P.B.U.H) stay truthful to the original message?
Is it mentioned why God protected the Qur'an from corruption but not the Bible?
Is there any value in reading the Torah, Psalms, Prophets, and Gospel today?
Basically I'd say yes there's a value as they include information which isn't in the Quran. And in whatever is related to good deeds and behavior they would have a similar level/meaning as the Quran has. For a Muslim it is recommended that he has good knowledge of the Quran to be able to judge, what is corrupt and what isn't.
I recommend you to read this Posts as they might be relevant (I've not checked them to 100%):
Are Muslims encouraged to read the Bible and/or the Torah? Why or why not?
How does Islam view the Christian New Testament?
Why didn't Allah send the complete book first?
